Reland "Removing dead ServiceManager constants"

This is a reland of 66630f37.

Reverted in 1764333 for breaking linux-chromeos-rel.
This change incorporates the fix.

Original change's description:
> Removing dead ServiceManager constants
>
> Since ServiceManager went away, we're clearing up some leftover
> CupsProxyService constants. Trivial change.
>
> Bug: chromium:945409
> Test: confirmed service behavior unchanged
> Change-Id: I45ffdeb3b0ad1990798a4a4d5115e791a6eefbfe
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1764430
> Auto-Submit: Luum Habtemariam <luum@chromium.org>
> Reviewed-by: Ken Rockot <rockot@google.com>
> Reviewed-by: Daniel Cheng <dcheng@chromium.org>
> Commit-Queue: Daniel Cheng <dcheng@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#689164}

Bug: chromium:945409, chromium:996442
Change-Id: Ie2d2a835787d5d3c178ee632f00031f30746d1ce
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1764525Reviewed-by: Ken Rockot <rockot@google.com>
Reviewed-by: Sean Kau <skau@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Luum Habtemariam <luum@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690400}

chrome/browser/chromeos/printing/cups_proxy_service_manager.cc

@@ -10,7 +10,6 @@
 #include "chrome/browser/chromeos/printing/cups_proxy_service_delegate_impl.h"
 #include "chrome/browser/profiles/profile_manager.h"
 #include "chrome/common/chrome_features.h"
-#include "chrome/services/cups_proxy/public/mojom/constants.mojom.h"
 #include "chromeos/dbus/cups_proxy/cups_proxy_client.h"
 #include "content/public/browser/browser_context.h"
 #include "services/service_manager/public/cpp/connector.h"

chrome/services/cups_proxy/public/mojom/BUILD.gn

@@ -7,7 +7,6 @@ import("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("mojom") {
   sources = [
-    "constants.mojom",
     "proxy.mojom",
   ]
 

chrome/services/cups_proxy/public/mojom/constants.mojom

-// Copyright 2019 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-module chromeos.printing.mojom;
-
-// Service id.
-const string kCupsProxyServiceName = "cups_proxy";
-
-// StartCupsProxyService capability id.
-const string kStartCupsProxyServiceCapability = "start_cups_proxy_service";